1. Field of the Invention
The present invention relates to a signal converter used in a tuning device for a radio receiver, a television receiver and the like.
2. Prior Art
A ferromagnetic resonator, for example, a YIG (YIG in the specification including yttrium iron garnet together with various additives) resonator using YIG as a ferromagnetic resonance element, is advantageous in that Q of resonance characteristics is high in the microwave band and compact construction is possible because the resonance frequency is not varied by the volume of the YIG element, and the resonance frequency can be varied linearly over a wide band region by varying the bias magnetic field applied to the element. Consequently, in recent years, in a receiver of high frequency signal such as microwave, a magnetic resonance element of YIG monocrystalline sphere or a YIG monocrystalline plate has been used as a resonance circuit for a tuning circuit. Such construction is proposed in Japanese unexamined patent applications laid-open No. 137609/1975, No. 78201/1975, No. 60402/1974 and No. 71215/1975.
When a signal converter for a tuning circuit is composed of a filter circuit and a local oscillator using a YIG resonator, the filter circuit and the local oscillator must have coincident resonance characteristics. If the YIG monocrystalline sphere or plate is used, however, it cannot be manufactured uniformly and precisely in the shape and size and dispersion is apt to occur in its characteristics. Consequently, the resonance characteristics are not coincident between the filter circuit and the local oscillator and therefore tracking errors may occur. In order to correct such errors, a special circuit construction such as disclosed in the patent application laid-open No. 137609/1975 is required and thus the whole circuit construction becomes complicated.